For the meringue: Heat 1-inch water in a 4-quart saucepan, covered, over high heat until it simmers, about 8 minutes. Decrease the heat to maintain a simmer.
Combine the sugar and whites in the bowl of a stand mixer and set over the simmering water. Whisk continually until the sugar has dissolved and the mixture lightens in color, doubles in volume, and reaches 140 degrees F, 3 to 4 minutes.
Place the bowl on the stand mixer and, using the whisk attachment, whisk on high until stiff peaks form and the meringue has cooled slightly, approximately 7 minutes. Set aside.
To form and cook the oeufs: Place the milk, 2 tablespoons of the sugar, and vanilla bean into a 2-quart saucier, cover, and set over high heat. Bring the mixture just to a simmer, approximately 5 minutes. Uncover, decrease the heat to low, and maintain a temperature of 180 to 190 degrees F.
Use a 1-1/2 ounce disher to scoop the meringue, 4 at a time, into the milk, and poach for 3 minutes. Gently flip with a slotted spoon and cook until firm to the touch, another 3 minutes. Remove to a tea towel-lined half sheet pan. Repeat with the remaining meringue mixture, maintaining the temperature of the milk to make sure it does not boil. Hold the meringues at room temperature until ready to serve, or store in an airtight container over night. (Can be made a day ahead and stored, and kept covered in the refrigerator.) Decrease the heat to low and keep the milk warm in the pan.
To finish the neige: Whisk together the egg yolks, kosher salt, and remaining 2 tablespoons sugar in a medium mixing bowl until well combined and slightly lightened in color, approximately 2 minutes. Slowly add half of the milk mixture, whisking constantly. Return the entire mixture to the saucepan over low heat, and stir frequently with a spatula or wooden spoon until the sauce has thickened slightly and coats the back of the spatula or spoon, 8 to 9 minutes. You should be able to draw a line on the back of the utensil with your finger.
Strain the sauce through the fine mesh strainer into a bowl set over an ice water bath and stir until chilled. (Can be made a day ahead and stored, covered in the refrigerator.)
To serve: Spoon a small amount of sauce into the center of a shallow bowl and top with 2 oeufs. Sprinkle on pistachios, if desired.